Dispensing systems are used widely in the semiconductor industry for a range of applications. These applications include underfill processes, encapsulation and dispensing of adhesive substances onto circuit boards for mounting components. Accuracy and precision of dispensing is becoming more important because sizes of semiconductor packages are gradually decreasing. In addition, automated measurement and adjustment of dispensed quantities with computerized control systems is becoming the norm to reduce human intervention and increase speed. As a result, accurate automated calibration systems are required.
For example, U.S. Pat. No. 5,906,682 for a “Flip Chip Underfill System and Method” discloses a calibration system which collects and measures an amount of material dispensed during a calibration routine against a target quantity. The weight of dispensed material at a given time is measured with weighing scales in order to adjust the dispensing system to achieve the target quantity. However, weighing scales are sensitive to changes in temperature so that the measured weight may vary with the temperature of air or the temperature of the sample. When small amounts of material are dispensed, such as to the order of 2 mg, such sensitive scales are only available at a relatively high cost. Furthermore, material used in a dispensing process is conventionally polymeric and fluid by nature, and measured in terms of cavity volume of encapsulation in a device, rather than in terms of units of weight. Although weight can be converted into volume for a specific type of material, the specific gravity of polymeric material will vary in space, resulting in dispensing errors in addition to the errors inherent upon conversion.
Another example is U.S. Pat. No. 6,412,328 entitled “Method and Apparatus for Measuring the Size of Drops of a Viscous Material Dispensed from a Dispensing System”. It discloses a dispensing apparatus having a housing, a dispenser that dispenses a quantity of viscous material, a measuring apparatus having a bottom plate to receive the viscous material, a top plate that is positioned over the bottom plate after the viscous material has been dispensed, and a compressing apparatus that compresses the material between the top and bottom plates. The quantity of material dispensed is determined by viewing the compressed material, and then multiplying the area of the compressed material by the distance between the two plates, which may be equivalent to the height of a gap shim between the two plates.
This method is time-consuming, in that mechanical manipulation of the various devices during calibration results in the wastage of a large proportion of calibration time. These steps of mechanical manipulation, including placement of a top pressure glass slide and plate by the dispensing system, locking using force rods, further compression of the plates with an air cylinder and finally the measurement by a viewing system are complicated. Thus, productivity lost from increased operation time of the dispensing device.
A further problem with the above prior art dispensing systems is that they are limited to calibration of a dispenser adapted for dot dispensing. Other modes of dispensation, such as line dispensing cannot be properly simulated and worked using this tool. Most of the dispensing systems used in surface mount technology require line dispensing within a heated environment, such as dam-fill encapsulation and flip-chip under-filling. Thus, the above methods do not provide an optimal solution to dispensing system calibration in a real environment.